In many areas of communications technology and in particular in mobile radio systems, a receiver must be synchronized to a transmitter before a connection is set up. For this purpose, the transmitter generally periodically transmits a specific sequence, which is known in the receiver and is also referred to as a sync word. This sequence is searched for in the incoming data stream in the receiver. When the sequence is found in the incoming data stream, the sought synchronization time is obtained from the timing of the detected sequence.
The data stream is typically subdivided in the transmitter into so-called slots (time slots) with a fixed number of bits (which are also referred to as chips when the spreading code method is used). The sync word which must be identified by means of a suitable method in the receiver is located at the start of a slot such as this. The expression slot synchronization is used in this case.
FIG. 1 shows a known procedure for determination of the slot start by means of a matched filter (search filter). The sampling time period is annotated T, while k denotes the time index (that is to say the discrete time). The sample values of the in-phase and quadrature components of the baseband signal x(kT) are supplied as an input signal 1 to an amplitude control means AGC (Automatic Gain Control) 2. The signal 3 emitted from the AGC 2 is passed to a low-pass filter TP 4. A signal 6 with a real amplitude is produced by magnitude formation (addition of the squares of the signal components) in a unit 5. This signal 6 is filtered by a matched filter 7. The impulse response of this filter 7 corresponds to the complex-conjugate sync word reflected in the time domain. The matched filter 7 produces a result g(kT) for each sample value x(kT). The maximum value occurs, in a similar way to that in the case of a correlator, at the filter output when the slot start has been found (minus the latency of the filter 7). The filtered signal g(kT) for all the sampling time kT, for example k=1, . . . , 2560, . . . , 5120, provided that 5120 samples are taken within one slot, is thus referred to as the correlation response 8.
A search for the maximum is carried out in a unit 9 within an observation interval, which corresponds to the slot duration, in order to detect correlation peaks in the correlation response 8. The maximum Umax and the associated time Tmax are passed to an evaluation unit 10. The evaluation unit 10 compares the detected maximum Umax with a specific, previously defined threshold value. The evaluation result 11 is signalled to a decision maker 12. If the threshold value has been exceeded, the decision maker 12 decides that a sync word has been detected at the time Tmax.
Since the mobile radio channel is not a static channel, it is not sufficient to carry out the method described above for only a single slot. A number of slots typically have to be processed in order to make an error-free decision on the slot boundaries. In consequence, intermediate results of the correlation responses 8 must be stored over a number of slot intervals and must be accumulated in the evaluation unit 10 before comparison with the threshold value. The threshold value level determines the yield and confidence of the method. The higher the preset threshold value in the evaluation unit 10 the fewer sync words are detected but, furthermore, the lower is the probability of an incorrect decision based on a sync word being simulated by interference.
The slot length in the UMTS (Universal Mobile Telecommunication System) Standard is 2560 chips (an in-phase and quadrature component in each case). An oversampling factor of 2 is normally used. Up to 10240 sample values therefore have to be stored, based on a typical resolution of 8 bits per slot.
A multistage evaluation method may be used in order to reduce the amount of memory. The principle of multistage evaluation is based on the idea of a subset of possible times for the slot start in different (mobile radio) cells being determined first of all during a initial selection process, for each slot. Only the times included in this subset are processed further and are possible candidates for the slot start of one or more cells. The slot start of each cell is then determined in a second selection step, which is more accurate and has a higher resolution than the first selection step.
With regard to the example of the UMTS standard, it is possible, by way of example, to provide for the 5120 correlation values emitted from the matched filter for each slot (for one signal component with double oversampling) to be restricted to a subset of, for example, 1280 correlation values. This initial selection process can be carried out with relatively little effort, but itself reduces the possible synchronization times by a factor of 4. The second selection step requires considerably greater accuracy (since only that time index at which the slot start is located must be determined from the 1280 time indices), but it need be carried out only for the preselected 1280 time indices k.
One difficulty with a multistage method such as this is that the two selection steps (initial selection and final selection) must be matched to one another as accurately as possible. In practice, the maximum size of the subset which can be processed in the second selection step is limited by hardware requirements (memory size, clocking etc). If the number of possible time indices in the initial selection process is greater than this subset, it is not possible to take account of all the preselected time indices in the second selection step. This results in the loss of information which was obtained in the first selection step. If, on the other hand, the number of time indices calculated in the first selection step is considerably less than the size of the subset (for example 1280 correlation values), the capacity of the second selection step is not utilized. Furthermore, during the initial selection process, there is a risk of this preselection process being excessively strict bearing in mind the low accuracy, thus likewise reducing the amount of information available for the second selection step. It is therefore important for the initial selection process to be carried out so as to just exhaust the capacity of the subsequent second selection step.